Catalytic converter
To comply with the U.S. Environmental Protection Agency's stricter regulation of exhaust emissions, most gasoline-powered vehicles starting with the 1975 model year are equipped with catalytic converters.
Although catalytic converters are most commonly applied to exhaust systems in automobiles, they are also used on electrical generators, forklifts, mining equipment, trucks, buses, locomotives, motorcycles, and on ships.
[8] In the mid-1950s, he began research to develop catalytic converters for gasoline engines used on cars and was awarded United States Patent 2,742,437 for his work.
To comply with the U.S. Environmental Protection Agency's new exhaust emissions regulations, most gasoline-powered vehicles manufactured from 1975 onwards are equipped with catalytic converters.
[13] To lower harmful NOx emissions, a twin-catalyst system was developed in the 1970s – this added a separate (rhodium/platinum) catalyst which reduced NOx ahead of the air pump, after which a two-way catalytic converter (palladium/platinum) removed HC and CO.[14] This cumbersome and expensive system was soon made redundant, after it was noted that under some conditions the initial catalyst also removed HC and CO.
[14] William C. Pfefferle developed a catalytic combustor for gas turbines in the early 1970s, allowing combustion without significant formation of nitrogen oxides and carbon monoxide.
An unregulated three-way converter features the same chemical processes but without the oxygen sensor, which meant higher NOx emissions, particularly under partial loads.
Then the air–fuel mixture is brought back to slightly rich, at a small penalty in CO and HC oxidation efficiency, and the cycle repeats.
The control system adjusts the rate of fuel being injected into the engine based on this signal to keep the air–fuel ratio near the stoichiometric point in order to maximize the catalyst conversion efficiency.
These converters often operate at 90 percent efficiency, virtually eliminating diesel odor and helping reduce visible particulates.
[citation needed] This led to an inventory runup by engine manufacturers in late 2006 so they could continue selling pre-DPF vehicles well into 2007.
The converter heats up quickly, due to its exposure to the very hot exhaust gases, allowing it to reduce undesirable emissions during the engine warm-up period.
An upstream air injection point, ahead of the catalytic converter, is also sometimes present to provide additional oxygen only during the engine warm up period.
This technique reduces the engine runtime needed for the catalytic converter to reach its "light-off" or operating temperature.
Oxygen sensors monitor the exhaust oxygen content before and after the catalytic converter, and the engine control unit uses this information to adjust the fuel injection so as to prevent the first (NOx reduction) catalyst from becoming oxygen-loaded, while simultaneously ensuring the second (HC and CO oxidation) catalyst is sufficiently oxygen-saturated.
Other common catalyst poisons include sulfur, manganese (originating primarily from the gasoline additive MMT), and silicon, which can enter the exhaust stream if the engine has a leak that allows coolant into the combustion chamber.
Depending on the contaminant, catalyst poisoning can sometimes be reversed by running the engine under a very heavy load for an extended period of time.
[29] Any condition that causes abnormally high levels of unburned hydrocarbons (raw or partially burnt fuel or oils) to reach the converter will tend to significantly elevate its temperature bringing the risk of a meltdown of the substrate and resultant catalytic deactivation and severe exhaust restriction.
These conditions include failure of the upstream components of the exhaust system (manifold or header assembly and associated clamps susceptible to rust, corrosion or fatigue such as the exhaust manifold splintering after repeated heat cycling), ignition system (e.g., coil packs, primary ignition components, distributor cap, wires, ignition coil and spark plugs) or damaged fuel system components (e.g., fuel injectors, fuel pressure regulator, and associated sensors).
The automotive aftermarket supplies high-flow converters for vehicles with upgraded engines, or whose owners prefer an exhaust system with larger-than-stock capacity.
[34] Catalytic converters have been mandatory on all new gasoline cars sold in the European Union and the United Kingdom since January 1, 1993 in order to comply with the Euro 1 emission standards.
[32] Carburetors on pre-1981 vehicles without feedback fuel-air mixture control could easily provide too much fuel to the engine, which could cause the catalytic converter to overheat and potentially ignite flammable materials under the car.
A pre-cat is most commonly used by an auto manufacturer when trying to attain the Ultra Low Emissions Vehicle (ULEV) rating, such as on the Toyota MR2 Roadster.
However, they also have some shortcomings in use, and also adverse environmental effects in production: Because of the external location and the use of valuable precious metals including platinum, palladium and rhodium, catalytic converters are a target for thieves.
The problem is especially common among late-model pickup trucks and truck-based SUVs, because of their high ground clearance and easily removed bolt-on catalytic converters.
[46][47][48] Pipecutters are often used to quietly remove the converter[49][50] but other tools such as a portable reciprocating saw can damage other components of the car, such as the alternator, wiring or fuel lines, with potentially dangerous consequences.
[59] In 2021 a trend emerged in the Democratic Republic of the Congo where catalytic converters were alleged to be stolen for use in illicit street drug production.
[61] Various jurisdictions now require on-board diagnostics to monitor the function and condition of the emissions-control system, including the catalytic converter.
[citation needed] The oxygen sensor is the basis of the closed-loop control system on a spark-ignited rich-burn engine; however, it is also used for diagnostics.
If both sensors show the same output or the rear O2 is "switching", the computer recognizes that the catalytic converter either is not functioning or has been removed, and will operate a malfunction indicator lamp and affect engine performance.
